[vhawk01]

Probability is what is to stop an animal from ever being born with enough mutations to be considered a different species. Actually, only probability+our definition of species. I think it is fairly safe to assume that never in the history of life have two organisms of a single species both randomly mutated in such a way as to be considered entirely different species, at the same approximate time, and mutated in such a similar way that they were capable of mating, and they found each other, and they mated, and their offspring survived.

This is a little different for organisms that reproduce asexually, I suppose.